1. Field of the Invention
The invention relates to improvements in phase-separation members, in particular solid-liquid industrial separation, filter media such as filter cloths or filter belts, for all recongnised pressure and vacuum filtrations systems, such as rotary drum filters, belt filter presses, etc. The term phase-separation members also includes papermaking fabrics such as forming fabrics, press felts, dryer fabrics or transfer fabrics. The invention also relates to corrugator belts and conveyor belts. These members are preferably water permeable.
2. Prior Art
It is known for such members particularly but not exclusively in the case of filters and papermaking fabrics, to provide a substrate, for example of a woven fabric, a spiral link fabric, a sintered sheet, a needlefelt or nonwoven textile, or a porous film. Such members are often made from or include fibres or particles of a low surface energy material such as a polyolefin, typically polypropylene.
To improve properties of the filter or papermaking fabric, such as filter cake release, it is desirable to be able to coat the substrate with a fluoropolymer such as PTFE. However, low surface energy materials such as polyolefins bond very poorly to fluoropolymers, and as a result durable release coatings are difficult to achieve on substrates of the kind mentioned.
We have previously proposed a process for filtering kaolin particles, in GB-A-2316015, using a filter comprising a fabric substrate coated or impregnated with a coagulated polymer. We have also proposed, in GB-A-2, 288, 755 a coated filter fabric comprising a cloth impregnated with a coagulated polymer latex which is coagulated in situ after impregnation in steam. These disclosures do not consider any problem other than provision of a microporous polymer medium within the voids of a fabric substrate, and do not suggest any solution to the problem of bonding a fluoropolymer coating to a low surface energy material substrate. Also in WO98/07925 we disclose a filter etc fabric which may be rendered porous by preinclusion of hollow yarns or fibres which may be opened by abrasion, to provide passages in the fabric. It is suggested that the fabric may be encapsulated in a polymer although it is not suggested that a coagulatable or microporous material be used, and separately that fluoropolymer may be used to improve non-stick and contaminant resistant properties. It is not suggested that there is any problem involved with the use of fluoropolymer coatings, or that the use of a microporous layer might mitigate this.
An object of the invention is to provide an improved phase-separation member or the like in which a durable bond is achievable between a low-surface energy substrate material, and a fluoropolymer.
According to the invention a phase-separation member or the like comprises a porous substrate of a low surface energy material having void spaces therein, and a fluoropolymer layer applied to at least one outer surface of the member, characterised in that a microporous polymer material is provided which at least partially impregnates said porous substrate, and in that the fluoropolymer layer is applied to an outer surface of said microporous polymer material such that the fluoropolymer layer remains predominantly at such outer surface, the microporous polymer serving to provide a bond between the fluoropolymer layer and the low surface energy material of the porous substrate.
The microporous polymer may be a coagulated polymer which is coagulated during or after impregnation of the substrate thereby.
The microporous layer can be of any synthetic or natural polymer which can be dissolved in a solvent, for example a polyurethane, silicone, fluoroelastomer or rubber.
The porous substrate may comprise or include particles, yarns or fibres of a low surface energy material such as a polyolefin, especially polypropylene. The porous substrate may be in the form of any of the substrates listed hereinbefore, i.e. woven or nonwoven fabric, knitted structures, needlefelt fabric, porous film, sintered sheet of metal or synthetic particles or fibres, or spiral link fabric.
All the above substrate structures include void space into which the coagulated polymer can at least partially penetrate. Preferably the substrate structure is impregnated to a substantial extent, e.g. to half or more than half the thickness of the substrate and provides a coating not only over the substrate but within the void space, of the yarns, fires, or particles forming the substrate to provide a filter medium of much finer pore size than would be provided by the substrate unaided. Advantageously, the coagulated polymer impregnates the substrate, i.e. provide a coating on both major surfaces as a well as impregnating the substrate.
The fluoropolymer used in coagulation or to coat the substrate after impregnation of the latter with the coagulated polymer may comprise a synthetic fluorinated elastomer such as polymers or copolymers of vinylidene fluoride; pentafluoropropene; tetrafluoroethylene; hexafluoropropene; e.g. vinylidene fluoride-pentafluoropropene-tetrafluoroethylene terpolymer, or vinylidene fluoride-hexafluoropropene-tetrafluoroethylene terpolymer. Fluoropolymers such as tetrafluoroethylene PTFE or other fluoro-alkene polymers however may be used.
The coagulatable polymer may be a relatively low viscosity material, in the range 300-1000 cP, e.g. of about 500 cP, and have a relatively high solids content. The low viscosity enables the polymer to penetrate substantially into the substrate structure, entering into the voids or interstices between fibres, yarns or particles making up the substrate.
The coating and impregnating layer of coagulatable polymer may be applied to the substrate as the polymer is coagulating, for example using DMF in a 5-30% solids solution. The coagulated polymer is typically a low surface energy polymer.
Coagulation may be achieved by heating the impregnated coated textile substrate in the presence of a heat coagulant. Suitable heat coagulants include vinyl alkyl ethers and derivatives thereof; polyacetals; polythio ethers; poly (ethylene oxide) and derivatives thereof; and poly (propylene/ethylene oxide) and derivatives thereof. Heating to a temperature of about 70xc2x0 C. is sufficient to effect coagulation.
An alternative method of coagulation is by adding a suitable electrolyte and/or varying the pH of the polymer latex. For example, with cationic polymers, coagulation may occur at an alkaline pH and for anionic polymers coagulation occurs at an acidic pH.
The coagulatable or coagulating polymer may be applied by any coating technique such as knife coating, dip-coating, screen printing or spraying, padding or using reverse roller techniques.
The fluoropolymer coating is in turn preferably applied to the outer surface of the coagulated polymer coated substrate by lick coating, spraying, foaming or paste spreading as a particulate dispersion, with for example 40-70 wt % solids and particle size 0.1-0.5 microns, onto the receiving surface and then the liquid component of the dispersion (which is preferably water for environmental reasons) is removed e.g. by evaporation pressing in a mangle, or suction into a slot, to leave a well-bonded low surface energy coating. Consolidation of the fluoropolymer coating can be improved by calendering the coated fabric to consolidate the structure, thereby improving retentivity (i.e. capture of filtrate particles) and smoothness (for better cake release).
The smooth fluoropolymer coating provides the microporous structure and any yarn knuckles or floats proud of said structure with enhanced abrasion resistance, as well as providing the fabric with good cake release properties. Filtrate particles are captured in the coagulated polymer forming the microporous structure.
Bodies such as hollow glass microbeads may be used to fill voids in the substrate, in place of or in addition to the coagulated polymer.
The coagulated polymer may be applied at a weight of 20-200 g/m2, producing a coating substrate (made from e.g. a polymolefin, polyester, polyamide, or PANO, with a weight of 50-2000 g/m2, before calendering.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.